In a conventional LCD, two pieces of transparent glass (for example, 300 mm (L) .times. 300 mm (W) .times. 0.5 mm to 1.1 mm (T)) are used as substrates for sandwiching a liquid crystal. An undercoat, ITO (indium Tin Oxide) electrodes as transparent electrodes, and an alignment film are formed respectively on the substrates, and the molecules of alignment film are then aligned.
Next, the two pieces of the glass substrates are placed to sandwich the ITO electrodes with a gap between them and bonded. The gap is formed for a liquid crystal layer. In the case of using such two glass substrates, the ITO electrodes can be formed on one of or both of the glass substrates.
Then, slits are made on the glass substrates so as to divide them into cells as liquid crystal display elements. Also, to expose the terminal sections of the ITO electrodes on one of the glass substrates, sections of the other glass substrate opposite the terminal sections are cut off.
With this manufacturing method, since it is possible to form numbers of cells in a pair of substrates having a predetermined gap therebetween at a time, cells having a uniform gap between the substrates are produced with precision. Besides, mass production permits a reduction in the manufacturing costs.
When dividing such glass substrates, firstly, scratches in the form of lines are made with a diamond cutter along predetermined dividing lines. Then, by applying a stress to the scratches with a breaking device, the glass substrates are broken along the dividing lines (breaking process).
In general, a glass substrate having a scratch in the shape of a line on its surface is easily broken when a stress is applied to the scratch. With this method, it is therefore possible to break the substrates along the scratches with precision.
In recent years, substrates produced from light-weight plastic are used for manufacturing a thinner and light-weight LCD. For the TN (Twisted Nematic)-type LCD, plastic films with a thickness of 0.1 mm to 0.3 mm, such as PES (Polyether Sulphone) or uniaxial oriented PET (Polyethylene Terephthalate) have been developed as substrates and mass-produced.
When cutting the substrates, a cutting device with a heat cutter is used or die cutting such as Thomson's die cutting method is adopted. In the case where such plastic films are used, however, it is impossible to cut only one of the two substrates if they are bonded. Therefore, to expose the terminal sections, cutting must be performed before bonding the two substrates.
Moreover, in recent years, it is known that plastic substrates produced from an acrylic resin or epoxy resin with the use of a mold having a polished surface are superior to those produced from the above-mentioned PES and uniaxial oriented PET in terms of the flatness of the substrate surface.
The acrylic resin and the epoxy resin are as stiff as they are used as lenses for glasses. If such resins are used to produce a substrate with a thickness of, for example, about 0.4 mm, the resulting substrate is brittle although it has some elasticity against a bending stress.
Therefore, unlike the glass substrates, even if stress is applied to split such a substrate along the scratches formed on the dividing lines thereon, it may not be broken along the dividing lines, deteriorating preciseness of dimensions. Thus, there is a possibility to produce defective products. In addition, the surfaces of the split sections tend to crack when stress is applied, and cracks tend to develop if a little stress is put, resulting in defective products.
Accordingly, the method used for cutting glass substrates can not be used when cutting substrates produced from an acrylic resin or epoxy resin to break them into cells and to expose terminal sections. Similarly, the method used for cutting a plastic film with a thickness of 0.1 to 0.3 mm can not be adopted when cutting such substrates.
Then, a cutting method including the dicing process using a substantially disk-shaped blade is suggested for cutting substrates having improved surface flatness, produced from hard and brittle plastic, such as acrylic resin and epoxy resin.
With this cutting method, the substrates are first placed on a movable stage as a worktable. Then, slits of a predetermined depth are formed on the substrates by rotating the blade while moving the stage so that the blade cuts the substrates along dividing lines. At this time, the depth of slit is determined so that the substrates are not completely cut off. The reason for this is to prevent the blade from coming into contact with and damaging the stage when cutting the substrates into cells and to prevent the terminal pattern from getting damaged when cutting the substrate to expose the terminal sections.
Next, the substrates are removed from the stage, and the uncut portion of the substrates is broken by applying stress to the substrates in a direction toward which the substrates are bent along the dividing lines. It is desirable to make the thickness of the uncut portion as small as possible when cutting the substrates with the blade.
With this cutting method, it is possible to divide the substrates without causing cracks because the sections of the substrates cut by the blade do not crack.
In contrast, with the above-mentioned conventional cutting method, the cut sections do not have smooth surfaces due to the following reasons.
With the present techniques for manufacturing the substrates, it is impossible to manufacture a substrate with a consistently uniform thickness. In reality, there is unevenness in the range of about .+-.70 .mu.m in the thickness of a substrate. Also, to prevent the blade from coming into contact with and damaging the stage, it is necessary to arrange the uncut portion to have a thickness which is greater than an appropriate value when cutting the substrates.
This arrangement causes an increase in the thickness of the portion to be split by bending. Moreover, since the thickness of the portion is not uniform, the substrates may not be split along the dividing lines, increasing the number of cracks on the split surfaces. Consequently, the number of defective products increases.
Besides, the frictional heat produced by the rotating blade and the substrate when cutting the substrate melts the surfaces of the cut sections and chips of the substrate caused by dicing. When the melted portions of the substrate become hard, a protruding burr is formed on each side of the openings of the slits along the dividing lines. When the liquid crystal display element is placed upside down so as to cut the other substrate which was located at the lower position before turning the liquid crystal display element, such burrs come into contact with the stage. This adversely affects the preciseness of the depth of slit made by the blade. Namely, the thickness of the portion to be uncut becomes uneven. It is therefore difficult to obtain smooth surfaces when the substrate is broken, increasing the number of defective products.